The 3rd-Generation Partnership Project (3GPP) is continuing development of the specifications for the Universal Terrestrial Radio Access Network (UTRAN). More particularly, work is ongoing to improve the end-user experience and performance in Release 11 of those specifications. These efforts include work to improve end-user experience and system performance in the CELL_FACH state, as part of a 3GPP Work Item called “Further Enhancements to CELL_FACH.”
CELL_FACH is a Radio Resource Control (RRC) state in which the end-user terminal (user equipment, or UE, in 3GPP terminology) is known to the network at the cell level (i.e., it has a cell ID) and has a layer 2 connection, but has no dedicated physical layer resources. Instead, the UE in CELL_FACH state must share common physical layer resources with other users in CELL_FACH state.
The Enhanced Dedicated Channel (E-DCH), which is an uplink packet-access channel, can be deployed so that it may be used by UEs in CELL_FACH state. More usually, E-DCH is used as a dedicated channel in CELL_DCH state, in which case a separate resource is allocated for each user. When E-DCH is used in CELL_FACH state, however, the system uses a pool of E-DCH resources that can each be temporarily assigned to a UE in CELL_FACH state.
This common pool of E-DCH resources is referred to herein as “common E-DCH resources.” E-DCH resources are normally managed by the Radio Network Controller (RNC), but the pool of common E-DCH resources is instead managed by the Node B (3GPP terminology for a base station). Configuration data specifying the E-DCH configurations are broadcasted to UEs in the cell.
A common E-DCH resource is defined as a particular combination of the following: an uplink scrambling code; an E-DCH Radio Network Temporary Identifier (E-RNTI); an F-DPCH code and timing offset; E-AGCH/E-RGCH/E-HICH codes and signatures; and parameters for use by the UE in uplink High-Speed Dedicated Physical Control Channel (HS-DPCCH) transmissions, such as power offsets and Channel Quality Report configuration information.
As of Release 10 of the 3GPP standards, the CELL_FACH state is commonly used to provide an efficient use of radio resources for UEs when data is arriving in bursts, with longer idle periods in between. The goals include both an efficient use of the UE's limited battery resources, as well as an efficient use of the network's radio resources. Ideally, an UE should be inactive between the bursts but should still be capable of swiftly moving into an active state when there are packets to send or receive. For this type of on-off traffic, the connection set-up latency and signaling load has a significant impact both on the preservation of the device battery and on the transmission quality perceived by the end user. In dormant periods, UEs are either sent to Idle state or are set to use configured Discontinuous Receive (DRX) schemes, to save battery energy.
Information specifying E-DCH resource configurations is broadcasted to UEs using SIB 5, which is a system information block sent over the BCH. Some of the broadcasted parameters, such as the Transmission Time Interval (TTI), are common for all common E-DCH resources.
The specifications for E-DCH as of Release 10 of the 3GPP specifications are rather rigid and do not allow flexible configurations. One example is the Transmission Time Interval (TTI) for common E-DCH resources. Currently, two different TTIs may be configured: either a 10-millisecond TTI or a 2-millisecond TTI may be configured. However, for coverage reasons, the network is likely to have some common E-DCH resources configured with a 10-millisecond TTI. As specified today, this implies that all resources must the same TTI. However, UEs in good radio conditions, e.g., in so-called hot spots, could make good use of common E-DCH resource with the shorter (2-millisecond) TTI. A shorter TTI improves both uplink throughput and network capacity, since each resource is occupied for less time. Concurrent deployment of 2-millisecond and 10-millisecond TTIs will thus provide the network with the flexibility to make an effective and optimal utilization of the common E-DCH resources.
Accordingly, as part of the 3GPP Release 11 Work Item “Further Enhancements to CELL_FACH,” 3GPP has decided to introduce support for concurrent deployment of 2-millisecond and 10-millisecond TTIs in a cell, for E-DCH transmission in CELL_FACH state and Idle Mode. According to the initial proposals, a requirement relating to UE power headroom is broadcast in a cell. UEs that fulfill the requirement will use the common E-DCH with a 2-millisecond TTI, while those not fulfilling the requirement will use the common E-DCH with a 10-millisecond TTI.
While concurrent deployment 2-millisecond and 10-millisecond TTI in a cell for E-DCH transmission in CELL_FACH state and Idle Mode offers opportunities for improved utilization of E-DCH resources, further improvements to network procedures and signaling are needed to take full advantage of these opportunities.